I have a guitar-playing client who frequently uses an octaver very loudly through his guitar amp for large sections of his shows. Needless to say, he blows speakers in his amps often.

I was thinking of building a cabinet with a couple guitar speakers, as well as a bass driver and a passive crossover that passes all frequencies < 80hz to the bass driver. This should protect his guitar speakers, and might even sound better. Is this possible/wise?

generally there are two ways to blow a speaker , one is to feed the speaker too much power which can blow it and the other is to run the amp into clipping which can send DC to the speaker which blows it .......

if he is blowing the speakers from too much power then he needs a higher power speaker if he is blowing the speaker from clipping the amp and sending dc to the speaker then you can put a high value Capacitor in series with the amp output which will block the dc , I would suggest about 10,000 uf at 100v .....